Gypsy moth (Lymantria dispar) egg masses have been observed hatching in Boylston, MA (Worcester County) the afternoon of 5/3/2018. (As reported by Dawn Davies of Tower Hill Botanic Garden, a scout for UMass Extension’s Landscape Message). Hatch was also observed in Southborough, MA the morning of 5/4/2018 by Mary Owen of UMass Extension. Egg masses at a site monitored in Amherst, MA (Hampshire County) also began to hatch on 5/4/2018. Those same eggs in Amherst were not yet hatching as of the day prior. While the spring of 2018 remained unseasonably cool for some time, the current summer-like temperatures have allowed many locations to quickly accumulate the growing degree days needed for gypsy moth egg hatch. (What are growing degree days? Read more at https://ag.umass.edu/landscape/fact-sheets/growing-degree-days-for-management-of-insect-pests-in-landscape). In 2017, gypsy moth egg hatch began around April 26, 2017 (observed in Belchertown, MA).

Anyone monitoring gypsy moth egg hatch should check local egg masses, particularly in locations where Amelanchier (shadbush) are blooming and growing degree-day accumulations are at or above 90-100 GDD’s, using a developmental threshold of 50°F. Egg masses are approximately 1.5 inches in length and can be a tannish-brown color and spongey as they are covered in hairs from the female moth who laid the eggs in 2017. A single egg mass can hold 500-1000 eggs. Tiny, hairy, dark-colored caterpillars may be observed on top of or nearby these egg masses.

After egg hatch, groups of tiny gypsy moth caterpillars may remain on their egg mass just before crawling to the canopy of their host plant, where they can disperse using a technique known as “ballooning”. Ballooning occurs when very young caterpillars spin a silken thread and catch the wind to blow onto a new host plant once the thread breaks. This method of dispersal can lead to host plants becoming defoliated that previously did not have egg masses directly on them. Egg masses may be present on nearby oaks, for example, and provide a local population of caterpillars.

Gypsy moth is a non-native, invasive insect in North America. Gypsy moth will feed on the leaves of and defoliate oak (preferred), maple, birch, poplar, willow, apple, and other deciduous plants. When their preferred hosts have been defoliated and gypsy moth populations are high, they will eat the needles of conifers such as eastern white pine, spruce, and hemlock. Such behavior was observed in 2016 and 2017.

Gypsy moth caterpillars defoliated over 350,000 acres across Massachusetts in 2016 and over 923,000 acres across MA in 2017 according to reports from the MA Department of Conservation and Recreation. For a map of where this occurred, visit: https://www.mass.gov/guides/gypsy-moth-in-massachusetts.

Luckily, the outlook for the 2018 season concerning gypsy moth is significantly better than it would have been if Entomophaga maimaiga, the gypsy-moth caterpillar killing fungus, didn’t outbreak in the gypsy moth caterpillar population in late June of 2017. At that time, many dead caterpillars were seen hanging from tree trunks and branches, killed by the fungus, which was aided by the wet spring weather earlier in the year. Need a reminder of what that looked like? Go to https://ag.umass.edu/landscape/news/gypsy-moth-caterpillars-dying-across-massachusetts.

Despite the fungal outbreak that swept through the 2017 caterpillar population, some lucky caterpillars survived to pupation and emerged as adult moths. (However, adults were present in far fewer numbers than would have existed without the fungus.) While it is very difficult to predict how much defoliation Massachusetts will see in 2018 due to gypsy moth caterpillar feeding, where egg masses have overwintered, pockets of defoliation could still occur in certain areas of the state this year. Thanks to the gypsy moth caterpillar-killing fungus, the population should be on the decline, and the outlook is far better than it was in 2017. However, we cannot expect the caterpillars to completely disappear from Massachusetts landscapes this season.